Cells grown in culture or obtained
ex-vivo can be easily cytospun or smeared on a slide. However
conditions are quite different from a tissue block, fixed and
embedded in paraffin.

To this end cell suspensions can be fixed, embedded in agar (an
inert material), and processed as a piece of tissue.

Live cell preparation.

Prepare 1% Agarose, cell culture
grade, in isoosmotic PBS. You need to boil to dissolve it in PBS.
Check for loss of water as vapor during boiling and replace with
distilled water.

Bring to approx 50 C°. The agar
solidify below this point.

Prepare your cells as a cell
suspension in minimal amount of medium, at room temperature.

To make a 0.5 ml agar block you may
need between 10 to 20x106 cells. You can scale this down
to as little as 0.5 x 106 in 50 µl but your cells will be
very sparse on section.

Prepare in advance 1.5 ml Eppendorf
tubes (or small PCR tubes for micro-preparations) to which you cut
off and discarded the conical bottom (!). Cap the tube.

Mix evenly and thoroughly your cells
with 0.5 ml of agar (or less) and very quickly transfer to the
capped inverted tube. No bubbles !! You may want also to cut the
pipette tip, so you have a larger opening.

Place the still molten agar and tube
on ice until is solid.

Then open carefully the
cap, and gently extract the agar cylinder from its base (the cap).
At this point you can cut the agar piece in two with a razor blade,
freeze half and fix in formalin the other. Your cells are still
alive and biochemically active at this point.

Fixed cell
preparation.

Proceed as above, but you fix the
cells before. Then you can use Agarose in PBS, regardless of
osmolarity.

Fixing the cells before embedding,
prevents movement of labile antigens or loss of short-lived
molecules.

Below is an example of a cell block
preparation.

Tricks.

It is very convenient to have on the same block a positive and a
negative control, but how to distinguish two semi-transparent block
of agar and identical-looking cells? Put a tiny amount of India Ink
(as your Surgical Pathologist) in the molten agar before embedding
the cells. Must be not more than a faint grey hue, enough to
distinguish the block with naked eye and at the microscope.